An endoscope typically includes an elongated shaft, which is suitable for insertion into a cavity, as well as a head, which can comprise junctions and operating elements as well as an eyepiece. In addition to, or in place of, the eyepiece, a junction for a video camera can be provided; frequently the eyepiece itself is configured for the connection of a video camera, which can be fastened for example to an eyecup. An optical system can be positioned within the shaft and head for retransmitting an endoscopic image from the distal end (that is, distant from the observer) of the endoscope to the proximal end (that is, close to the observer). For said purpose, an optical system can include in particular a distally mounted objective lens for recording the endoscope image, an image-relaying element and an ocular with eyepiece for visual observation of the retransmitted endoscopic image. The endoscope can be configured, in particular, with a rigid shaft and can comprise a rod lens arrangement as image retransmitter, but can also be configured semi-flexibly or flexibly with a glass fiber bundle as image retransmitter. The endoscope can comprise an illumination device for illuminating a cavity that is to be observed, and in some cases channels for endoscopic working instruments.
For hygienic reasons, endoscopes intended for medical purposes must be sterilizable. Sterilization today is primarily done by autoclaving, that is, by hot vapor at increased pressure, for example at 2 bar, and at a temperature of approximately 140 degrees C. However, optical systems of endoscopes are as a rule sensitive to hot vapor, which can penetrate during hot vapor sterilization and can result, for example, in glass corrosion. In addition, moisture that has penetrated into an optical system can result in coating of optical elements and thereby can impede functioning. For this reason, protective measures against penetration of hot vapor can be required, in particular the pressure conditions prevailing in the use of hot vapor sterilization. Such protective measures are also advantageous with other sterilization methods such as chemical sterilization as well as, for example, with technical applications in cavities that contain aggressive gaseous or liquid media.
In particular, the optical system can be enclosed in a vapor-proof tube that is of continuous construction or consists of a number of tubes or sheaths that are connected with one another and are vapor-proof. The proximal and distal ends of the tube in this case are each closed with cover glasses, which are connected in vapor-proof manner with the tube or with a sheath connected with the tube.
To produce a vapor-proof connection between a cover glass and the tube or with a sheath, it is a known practice to cement the cover glass with the tube or sheath, for example by spraying the cover glass with a synthetic substance, such as polyphenylsulfone (PPSU), which can also be used for example to construct the eyecup. PPSU, however, is not sufficiently resistant to chemicals used in disinfection and sterilization and can be discolored during the preparation. Other synthetics or cementing substances also have disadvantages concerning resistivity to sterilization and cannot always ensure a reproducible insulated connection, and/or may not sufficiently prevent or obstruct diffusion processes of moisture by the synthetic or cementing.
It is a known procedure from DE 37 40 417 A1, in case of a rigid endoscope lens system with a metallic tube that encloses the lenses and is closed on the distal end with a window, to coat the surrounding surface of the distal end window with a metallic film and to solder this metallic film with the tube inner surface.
According to WO 98/04948, an end window of an endoscope is cemented into an outer tube, inside which the optical system is enclosed, or into an intermediate part inserted into the outer tube, while the end window is additionally connected with the outer tube or the intermediate part by means of a soldered layer. For this purpose, a solderable ring-shaped layer is applied on at least one end surface of the end window.
Applying such a solderable metallic coating, however, constitutes additional expense. A desirable solution, therefore, would be not to perform the soldering with a prior applied metallic film but rather directly with the glass, which as a rule is a sapphire glass in this case. A cost-effective method for soldering sapphire glass is active soldering, in which a special solder, which in particular can contain silver, copper and titanium, is applied in a vacuum at a temperature of about 900 degrees C. or more. In this case, previous coating with a solderable layer is required. Under these conditions, however, an anti-reflecting coating, which is advantageous for improving the image quality at least on the inside of the cover glass, is destroyed. Such an anti-reflecting coating itself is not solderable. It is disclosed in U.S. Pat. No. 5,536,244 that an end window is soldered directly into a metallic frame that is inserted into the endoscope shaft and welded to it; no coating is mentioned.